MICROFICHE APPENDIX
This application includes a microfiche appendix consisting of 32 slides and 6,267 frames, which is a copy of the provisional application under which priority is claimed and updated source code.
1. Field of the Invention
This invention relates to phacoemulsification devices and, more particularly, to a method for controlling a phacoemulsification device.
2. Related Art
Ultrasonic probes have traditionally been used for phacoemulsification, namely, for rupturing of cataracts in the eye and for aspiration of the pieces of tissue disrupted. These ultrasonic probes must be carefully powered for proper operation. Operating the ultrasonic probe at its resonant frequency takes advantage of the resonant characteristics of the ultrasonic transducer. Resonance is defined as the phenomenon wherein a system is driven at or near one of its natural modes.
Accordingly, the prior art has focused on how to determine the resonant frequency of a transducer. Theoretically, this problem has been solved. A typical way of determining the resonant frequency of an ultrasonic transducer is to compare the phase angle between the voltage waveform applied to the ultrasonic transducer and the waveform of the current drawn by the transducer.
When voltage is applied to a circuit, current will flow through the circuit. When the voltage and current waveforms are viewed for a particular circuit, the current waveform will lag the voltage waveform if the circuit is inductive, and the voltage waveform will lag the current waveform if the circuit is capacitive. The time difference between the points when the current waveform and voltage waveform intersect the zero axis is measured in trigonometric terms by the phase angle .PHI.. For purely resistive circuits, .PHI. equals zero and the voltage in the current waveforms are said to be in phase. For purely inductive circuits, .PHI. equals 90.degree. and for purely capacitive circuits, .PHI. equals -90.degree. and the voltage in the current waveforms are said to be out of phase.
The presence of an inductive or capacitive reactance component in a load impedance will decrease the efficiency of power delivery of the system since only resistive components can actually dissipate power.
For circuits containing all three elements, resistors, inductors and capacitors, there will be some frequencies where the total impedance of the circuit will appear purely resistive even though the circuit contains reactive elements, i.e., the resistive elements plus the imaginary component caused by the presence of the inductive and capacitive elements. These frequencies are at or near the resonant and/or anti-resonant frequencies.
Therefore, in theory, one method of determining the resonant frequencies of certain types of complex circuits is to apply an alternating voltage to the circuit and to vary the frequency until the phase angle .PHI. between the voltage and current is zero. The frequencies where this condition occurs are the actual resonant frequencies of that particular circuit. The resonant frequency is that frequency or frequencies at which the circuit response (i.e., admittance) is locally a maximum, and the anti-resonant frequency is that frequency or frequencies at which the response achieves a local minimum.
When driving a circuit having both resistive and reactive components, it is important to know the value of the phase angle .PHI. because the power delivered to a load is given by the following equation: EQU Power=VI cos (.PHI.)
where V is the voltage drop across the load impedance; I is the series current flowing through the load impedance; and cosine phi is the power factor of the circuit. Clearly, for a phase angle equal to zero, cosine (0) equals 1 and the transfer of power from the source to the circuit is at maximum. This situation exists where a purely resistive load exists.
As these theoretical principles are practically applied, problems have been encountered. Specifically, as environmental conditions such as temperature, time, etc., change, the characteristics of the probe changes. These changes are reflected as changes in the values of the various resistive and reactive components of the ultrasonic probe electrical model of FIG. 1. In other words, as the environmental factors change, the mechanical resonant frequency of ultrasonic probe changes also. To solve this problem, there has been a direction in the prior art to provide a phase locked circuit to ensure that the phase angle of the system, .PHI., will be zero, such as for example in U.S. Pat. Nos. 5,446,416; 5,210,509; 5,097,219; 5,072,195; 4,973,876; 4,484,154; and 4,114,110.
However, loading on the transducer will have a damping effect on the vibrations of the transducer. In other words, the load may dampen the vibrations of the transducer. When this condition occurs, the resonant frequency may change and phase angle .PHI. will longer be zero and the transfer of power will no longer be optimum. Therefore, unless provisions are made in the circuit to alter the phase angle .PHI., optimum power transfer cannot be achieved.
Accordingly, methods other than locking the phase angle .PHI. have been explored such as using a tunable inductor in a control system to cancel out the capacitive reactants of the load impedance presented by the ultrasonic transducer, such as that disclosed in U.S. Pat. Nos. 4,970,656; and 4,954,960. Alternatively, using the admittance of the ultrasonic transducer as the tuning parameter rather than the phase angle has also been explored in U.S. Pat. No. 5,431,664.
Approaching this problem from a purely output power standpoint has also been explored in U.S. Pat. No. 5,331,951 in which the actual electrical power supplied to the drive circuit is examined and the supply voltage is varied after comparing the electrical power supplied with the desired transducer power level. Tangentially, this patent also addresses a way to substantially minimize the power amplifier's power consumption by providing a boost regulator for supplying voltage to the amplifier.
In yet another approach, phase-regulated power and frequency control is utilized, such as in U.S. Pat. No. 4,849,872. Therein the initial resonance frequency of the ultrasonic transducer is determined and a capacitive phase angle between the voltage waveform and current waveform is introduced and maintained so that by phase control of the phase control circuit, the operating frequency of the oscillator is reduced relative to the series resonance frequency of the transducer. The phase angle is typically maintained as a non-zero constant. Similarly, in U.S. Pat. No. 4,888,565, a power control feedback loop for monitoring the output signal and a frequency control feedback loop are utilized to provide maximum current. This approach relies on holding the mains current constant.
An electrical model of a ultrasonic phacoemulsification probe in the vicinity of resonance is provided in FIG. 1. The model has a voltage source 1401 connected to a 1130 picofarad capacitor 1402 connected in parallel to a series RLC circuit 1403, wherein the resistor is 220 ohms, the inductor is 1.708 henrys, and the capacitor is 18 picofarads.
When examining the apparent power resulting from the electrical model, the graphs of FIGS. 2 and 3 are obtained. As seen in these Figures, the apparent power peaks at 28.661 kHz with a phase angle of approximately -42 degrees. This is expected due to the parallel capacitance in RLC circuit 1403.
When examining the real power resulting from the electrical model, the graphs of FIGS. 4 and 5 are obtained. As seen in these Figures, the real power peaks correctly at 28.7 kHz, but the phase angle is approximately -24.5 degrees.
When a compensating inductor with a calculated value of 27.21 millihenrys is placed in ghost block 1404 of FIG. 1 to cancel the reactive component of FIG. 1 and the resultant apparent power and real power information is obtained as in FIGS. 6 and 7, the apparent power and the real power now both correctly peak at 28.7 kHz with a phase of approximately -0.5 degrees. Thus, it can be seen that the inductor in ghost block 1404 compensates the parallel capacitance 1402 and makes the circuit appear resistive (zero phase) at resonance. From these graphs, it is clear that the real power provides a more accurate view of resonance frequency, unless a compensating inductor is added near resonance. Accordingly, resonant frequency is defined herein as the frequency at which real power achieves a (local) maximum. However, apparent power may be used to determine the resonant frequency if the parallel capacitance is compensated at resonance. Apparent power provides an approximation of resonant frequency (frequency at which the local maximum occurs) if a compensating inductor compensates parallel capacitance 1402 near resonance.
Therefore, there is a need in the art to maximize the power output to an ultrasonic transducer which is responsive to both environmental changes as well as changes in loading, and yet which also does not necessarily require a fixed phase angle or a constant current.